Multi-band wireless transceiver systems have been proposed to address an increasing demand for bandwidth enhancement and flexibility of wireless telecommunication systems. Many different standards for such systems are currently in use around the world. Each of these standards sets forth specifications for various aspects of the communication link, such as the frequency of the transmitted signal, its spectral purity to avoid interference with other services and the scheme used for encoding information in the signal. Examples of different systems currently in use are the GSM900 mobile communication system popular in Europe and parts of Asia and the PCS1900 system in North America. Many other systems are used for various purposes, and most of these systems specify a different frequency for operation.
The use of different standards in different or even the same area on the globe creates challenges to system providers who would like to offer flexible systems capable of complying with a multitude of the standards in use. The increased flexibility of such multi-band communication systems can potentially provide various benefits to the end-user. For example, a dual-band cellular phone complying both with the GSM900 and the PCS1900 systems can use these services in North America, Asia and Europe, and its user would not be required to use a different phone when traveling outside its service area. Another application could be a communication system that uses different standards in the same area depending on outside circumstances such as to select the best frequency band that offers the best reception. Other possible applications are systems that use different frequency bands for different functionalities, such as equipment combining a GSM900 cellular phone with a global-positioning system (GPS) device. Many such applications are possible.
In wireless transmitters, signal amplifiers are often tuned for the frequency of intended operation. Tuning provides improved operation with respect to the gain provided by the amplifier, the spectral purity of the amplified signal, and the mode of operation (e.g. Class, A, Class AB, Class B and Class F, among others), all of which are designed to comply with the communication standards set for the intended use. Furthermore, tuning may also allow more efficient operation with respect to power consumption than can be achieved by broadband amplifiers.
Thus, communication systems designed to transmit signals at a multitude of frequencies must be able to amplify signals at each one of these frequencies. This is hereinafter called “multi-band” amplification.
Various design strategies have been disclosed in the art that ostensibly expand the functionality of power amplifiers that are designed into wireless transmitters. For example, U.S. Pat. No. 5,060,294 to Schwent et al. describes a dual-mode power amplifier. A dual-mode power amplifier is an amplifier operating in one of two different modes in a single frequency band. Such an amplifier, however, is not capable of operating in more than one frequency band, but rather is capable of being operated either in linear (analog) mode or saturated (digital) mode, thereby improving the utility of the amplifier for applications requiring differently modulated signals.
U.S. Pat. No. 5,774,017 to Adar describes a dual-band power amplifier, defined as an amplifier that tunes a signal at two frequencies of operation. This is accomplished by applying an external frequency-indicating control signal to the networks, and more specifically by (1) selectively coupling the signal to a variety of impedance networks; or (2) selectively coupling the signal to parts of the impedance networks; or (3) by changing component impedances in the impedance-tuning networks used in the amplifier, depending on the signal frequency.
In general, selective coupling can be achieved in several conventional ways. In one method, active components are used, namely, components that act as switches or variable conductors controlled by a signal indicating the frequency of signal. In a second method, filters connected in parallel are employed, such that each of these filters accept a signal at only one frequency and reject signals at all other frequencies. In this way, a signal is routed through only one of the filters depending on the frequency of the signal. This latter method is known as diplexing.
Unfortunately, designs involving techniques of selective coupling as mentioned above, however, suffer from several drawbacks. Techniques employing switched networks or diplexers use some or all components in the networks only for one frequency band, hence they tend to employ more components than is desirable. Reducing the components count can directly benefit a system's cost and reliability. Moreover, switches used to route the signal introduce losses while offering little other functionality. Finally, techniques using selectively coupled networks or selectively controlled component impedances, are inherently incapable of amplifying signals in more than one frequency band at the same time.
Thus it would be highly desirable to have a multi-band power amplifier design that minimizes component count and that does not rely on selective coupling or diplexing.
Recent advances in design methodology of dual-band receivers, and in particular of dual-band low-noise receiver amplifiers, have shown improvements in existing functionality of dual-band low-noise receiver amplifiers as well as introducing the possibility for concurrent dual-band operation by the use of inherently dual-resonant impedance networks instead of selective impedance network coupling or network impedance controlling schemes such as described in U.S. Pat. No. 5,774,017.
It is therefore an object of the invention to provide a switchless multi-resonant, multi-band power amplifier that can operate without any frequency indicating control signal schemes.